Glass-ceramic composition for recording disk substrate

ABSTRACT

A glass ceramics composition for recording disk substrate contains, essentially, expressed in terms of weight percent on the oxide basis, from 47.5 to 52.5 wt % of SiO 2 , from 10 to 30 wt % of Al 2 O 3 , from 10 to 30 wt % of MgO, and from 6.2 to 7.8 wt % of TiO 2 .

RELATED APPLICATION

This application is based on application No. 11-200448 filed in Japan, the content of which is hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a glass ceramic composition, more particularly, relates to the glass ceramic composition suitable for magnetic disk substrate.

DESCRIPTION OF THE PRIOR ART

Magnetic disks are mainly used as recording media of computers. Aluminum alloys have heretofore been used as the material of magnetic disk substrates. However, in the recent trend for a smaller size, a thinner thickness, and a higher recording density of magnetic disks, a higher surface flatness and a higher surface smoothness are increasingly desired. Aluminum alloys cannot satisfy the desire, and a material for magnetic disk substrates which can replace aluminum alloys is required. Thus, in particular, recent attention has been focused on the glass substrate for the disk because of its surface flatness and smoothness and excellent mechanical strength.

As glass substrates for disks for recording media, there have been proposed a chemically reinforced glass substrate having a surface reinforced by ion exchange or like method and a glass ceramics substrate on which a crystal component has been precipitated to reinforce the bonding. In recent years, the latter crystallized glass substrate in which a crystallite has been precipitated in glass by heat treatment has drawn particular attention because of its excellent strength and high productivity.

As recent requirements on the performance of a disk for a recording medium have been more stringent, a substrate material has also been required to have an increased strength related directly to the bending or warping of the disk during high-speed rotation. The strength can be represented by the elastic modulus ratio (=Young's modulus/specific gravity) of the substrate material. The elastic modulus ratio having a higher value indicates a higher mechanical strength. However, a glass-ceramics composition conventionally known has the problem that the productivity thereof is reduced significantly if the strength thereof is to be increased.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a glass ceramic composition which is suitable for use in an improved glass substrate for a recording medium.

Another object of the present invention is to provide a glass ceramic composition which has high productivity irrespective of its high elastic modulus ratio.

Still another object of the present invention is to provide a disk substrate for a recording medium which has high productivity irrespective of its high elastic modulus ratio.

Thus, the present invention provides a glass-ceramics composition consisting essentially, expressed in terms of weight percent on the oxide basis, of, from 47.5 to 52.5 wt % of SiO₂, from 10 to 30 wt % of Al₂O₃, from 10 to 30 wt % of MgO, and from 6.2 to 7.8 wt % of TiO₂.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a glass-ceramics composition consisting essentially, expressed in terms of weight percent on the oxide basis, of, from 47.5 to 52.5 wt % of SiO₂, from 10 to 30 wt % of Al₂O₃, from 10 to 30 wt % of MgO, and from 6.2 to 7.8 wt % of TiO₂.

In the composition, SiO₂ is a glass network former oxide. The melting properties deteriorate if the proportion thereof is lower than 47.5 wt %. If the proportion thereof exceeds 52.5 wt %, the composition becomes stable as glass so that the crystal is less likely to be precipitated.

Al₂O₃ is a glass intermediate oxide and a component of an aluminum borate crystal, which is a crystalline phase precipitated by heat treatment. If the proportion of Al₂O₃ is lower than 10 wt %, the crystal is precipitated in reduced quantity and a sufficient strength is not achieved. If the composition rate of Al₂O₃ exceeds 30 wt %, the melting temperature is increased and devitrification is more likely to occur.

MgO is a fluxing agent. MgO forms an aggregation of crystal grains. If the proportion of MgO is lower than 10 wt %, the range of operating temperatures is narrowed down and the chemical durability of a glass matrix phase is not improved. If the proportion of MgO exceeds 30%, another crystalline phase is precipitated so that it becomes difficult to achieve a desired strength.

TiO₂ is a fluxing agent. By adding TiO₂ serving as a fluxing agent has been added, production stability has been improved. If the proportion of TiO₂ is lower than 6.2 wt %, the melting properties deteriorate and the crystal is less likely to grow. If the proportion of TiO₂ exceeds 7.8 wt %, the crystallization is promoted rapidly so that the control of the crystallized state becomes difficult, the precipitated crystal is increased in size, and the crystalline phase becomes non-uniform. This prevents the obtention of an extremely small and uniform crystal structure and the obtention of a flat, smooth surface by polishing, which is required of the glass substrate as a disk substrate. Moreover, devitrification is more likely to occur during melt molding, which lowers productivity.

Besides the above-mentioned basic components, P₂O₅ as a fluxing agent and a nuclear forming agent can been added. By adding P₂O₅ is a fluxing agent and a nuclear forming agent for precipitating a silicate crystal, the crystal are uniformly precipitated over the entire glass. If the proportion of P₂O₅ is lower than 0.1 wt %, satisfactory nuclei are less likely to be formed so that crystal grains are increased in size or the crystal is precipitated non-uniformly. Consequently, an extremely small and uniform crystal structure is less likely to be obtained and a flat, smooth surface required of the glass substrate as a disk substrate cannot be obtained by polishing. If the proportion of P₂O₅ exceeds 5 wt %, the reactivity of the glass in a molten state to a filter medium is increased and the devitrifiability thereof is also increased, so that productivity during melt molding is reduced. In addition, the chemical durability is reduced, which may affect a magnetic film, while the stability in the polishing to cleaning steps is lowered.

Besides the above-mentioned basic components, Nb₂O₅ as a fluxing agent can been added. By adding Nb₂O₅ serving as a fluxing agent, production stability has been improved. If the proportion of Nb₂O₅ is lower than 0.1 wt %, the rigidity is not sufficiently improved. If the proportion of Nb₂O₅ exceeds 9 wt %, the crystallization of the glass becomes unstable and the precipitated crystalline phase cannot be controlled, so that desired characteristics are less likely to be obtained.

Besides the above-mentioned basic components, Ta₂O₅ as a fluxing agent can been added. By adding Ta₂O₅ serving as a fluxing agent, the melting properties and strength are improved, while the chemical durability of the glass matrix phase is improved. If the proportion of Ta₂O₅ is lower than 0.1 wt %, however, the rigidity is not sufficiently improved. If the proportion of Ta₂O₅ exceeds 9 wt %, the crystallization of the glass becomes unstable and the precipitated crystalline phase cannot be controlled, so that desired characteristics are less likely to be obtained.

Besides the above-mentioned basic components, Li₂O as a fluxing agent can been added. By adding Li₂O serving as a fluxing agent, production stability has been improved. If the proportion of Li₂O is lower than 0.1 wt %, the melting properties deteriorate. If the proportion of Li₂O exceeds 12 wt %, stability in the polishing to cleaning steps is degraded.

Besides the above-mentioned basic components, ZrO₂ as a glass modifying oxide can been added. By adding ZrO₂ serving as a glass modifying oxide, a glass nucleating agent functions effectively. If the proportion of ZrO₂ is lower than 0.1 wt %, satisfactory crystal nuclei are less likely to be formed so that crystal grains are increased in size and the crystal is precipitated non-uniformly. This prevents the obtention of an extremely small and uniform crystal structure and the obtention of a flat, smooth surface by polishing, which is required of the glass substrate as a disk substrate. In addition, the chemical durability and the migration resistance are reduced. This may affect a magnetic film and degrades stability in the polishing to cleaning steps. If the proportion of ZrO₂ exceeds 12 wt %, the melting temperature is increased and devitrification is more likely to occur during melt molding, which lowers productivity. Moreover, the precipitated crystalline phase changes so that desired characteristics are less likely to be obtained.

Besides the above-mentioned basic components, B₂O₃ as a former can been added. By adding B₂O₃ serving as a former, the phase splitting of the glass is promoted and the precipitation and growth of the crystal are promoted. If the proportion of B₂O₃ is lower than 0.1 wt %, the melting properties are not improved sufficiently. If the proportion of B₂O₃ exceeds 5 wt %, devitrification is more likely to occur and molding becomes difficult, while the crystal is increased in size, so that an extremely small crystal is no more obtained.

Besides the above-mentioned basic components, Y₂O₃ as a fluxing agent can been added. By adding Y₂O₃ serving as a fluxing agent, the rigidity has been improved. If the proportion of Y₂O₃ is lower than 0.1 wt %, however, the rigidity is not improved sufficiently. If the proportion of Y₂O₃ exceeds 9 wt %, the precipitation of the crystal is suppressed and a sufficient degree of crystallization is not achieved, so that desired characteristics are not achieved.

Besides the above-mentioned basic components, K₂O as a fluxing agent can been added. By adding K₂O serving as a fluxing agent, production stability has been improved. If the proportion of K₂O is lower than 0.1 wt %, however, the melting properties are not improved sufficiently. If the proportion of K₂O exceeds 5 wt %, the glass becomes stable and the crystallization is suppressed, while the chemical durability is reduced. This may affect a magnetic film and degrades stability in the polishing to cleaning steps.

Besides the above-mentioned basic components, Sb₂O₃ as a clarifier can been added. By adding Sb₂O₃ serving as a clarifier, production stability has been improved. If the proportion of Sb₂O₃ is lower than 0.1 wt %, however, a sufficient clarifying effect can not be achieved and productivity is lowered. If the proportion of Sb₂O₃ exceeds 5 wt %, the crystallization of the glass becomes unstable and the precipitated crystalline phase cannot be controlled so that required characteristics are less likely to be obtained.

Besides the above-mentioned basic components, ZnO as a fluxing agent can been added. By adding ZnO serving as a fluxing agent, it helps uniform precipitation of the crystal. If the proportion of ZnO is lower than 0.1 wt %, however, the uniformity of the crystal is not sufficiently improved. If the proportion of ZnO exceeds 5 wt %, the glass becomes stable and the crystallization is suppressed, so that required strength is less likely to be achieved.

Besides the above-mentioned basic components, La₂O₃ as a fluxing agent can been added. By adding La₂O₃ serving as a fluxing agent, the precipitation of the crystal is suppressed. If the proportion of La₂O₃ is lower than 0.1 wt %, however, the rigidity is not improved sufficiently. If the proportion of La₂O₃ exceeds 5 wt %, the crystallization of the glass becomes unstable and the precipitated crystalline phase cannot be controlled, so that required properties are less likely to be obtained.

Next, a description will be given to a fabrication method. Raw materials containing the main components of the glass substrate to be finally produced are sufficiently mixed in specified proportions. The resulting mixture is placed in a platinum crucible and caused to melt. The molten product is cast in a metal mold so that it is formed into a rough configuration and annealed to a room temperature. The molten product is then held at a specified temperature for a specified time during a primary treatment (heat treatment) such that crystal nuclei are formed. Subsequently, the molded mixture is held at a specified temperature for a specified time during a secondary heat treatment such that crystal nuclei grow. By slowly cooling the molded mixture, an objective crystallized glass is obtained.

NUMERICAL EXAMPLES

A description will be given next to specific numerical examples incorporating the embodiments. In TABLE 1 are shown: the proportions(unit: wt %) of materials composing the glasses of the first to fifth examples; the melting temperatures and times; the primary heat treatment temperatures and times; the secondary heat treatment temperatures and times; the main precipitated crystalline phases; the subordinate precipitated crystalline phases; the mean diameters of the crystal grains; the specific gravity s: the Young's moduli; and the specific moduli. Likewise, tshe glasses of the sixth to tenth examples are shown in TABLE 2. Likewise, the glasses of the eleventh to fifteenth examples are shown in TABLE 3. Likewise, the glasses of the sixteenth to twentieth examples are shown in TABLE 4.

TABLE 1 Example 1 Example 2 Example 3 Example 4 Example 5 SiO₂ 50.4 51.7 52.5 47.5 52.0 Al₂O₃ 6.4 11.4 17.3 19.0 26.5 MgO 13.6 14.0 23.8 24.5 13.0 P₂O₅ 1.4 1.5 0.2 1.2 Nb₂O₅ 2.0 Ta₂O₅ Li₂O 2.4 2.5 TiO₂ 6.8 7.0 6.2 7.8 6.5 ZrO₂ B₂O₃ Y₂O₃ 17.0 K₂O 1.5 1.6 Sb₂O₃ 0.4 0.4 ZnO La₂O₃ 10.1 Melting 1450 1450 1500 1500 1500 Temperature (° C.) Melting Time 2.50 3.00 3.00 3.00 3.00 (hours) Primary 660 690 690 690 690 Treatment Temperature (° C.) Primary 5.00 5.50 5.50 5.50 5.50 Treatment Time (hours) Secondary 820 820 840 840 840 Treatment Temperature (° C.) Secondary 5.50 5.50 6.00 6.00 6.00 Treatment Time (hours) Main Magnesium Magnesium Magnesium Magnesium Magnesium Crystalline alumina silicate alumina silicate alumina silicate alumina silicate alumina silicate Phase Sub Crystalline Rutil Rutil Rutil Rutil Rutil Phase Diameter of 0.05 0.05 0.05 0.05 0.05 Crystal Specific 2.92 2.81 2.96 2.90 3.02 Gravity (g/cm³) Yong's Modulus 102.85 97.51 105.85 100.85 105.85 Elastic 35.23 34.74 35.77 34.79 35.06 Modulus Ratio

TABLE 2 Example 6 Example 7 Example 8 Example 9 Example 10 SiO₂ 48.0 51.8 52.2 52.2 47.8 Al₂O₃ 18.0 19.2 22.0 27.2 18.0 MgO 23.0 20.3 13.0 12.0 20.7 P₂O₅ Nb₂O₅ 4.0 Ta₂O₅ 2.2 5.2 Li₂O 2.4 6.0 TiO₂ 7.0 6.5 7.6 6.2 7.5 ZrO₂ B₂O₃ Y₂O₃ K₂O Sb₂O₃ ZnO La₂O₃ Melting 1500 1500 1500 1500 1500 Temperature (° C.) Melting Time 3.00 3.00 3.00 3.00 3.00 (hours) Primary 690 690 690 690 690 Treatment Temperature (° C.) Primary 5.50 5.50 5.50 5.50 5.50 Treatment Time (hours) Secondary 840 840 840 840 840 Treatment Temperature (° C.) Secondary 6.00 6.00 6.00 6.00 6.00 Treatment Time (hours) Main Magnesium Magnesium Magnesium Magnesium Magnesium Crystalline alumina silicate alumina silicate alumina silicate alumina silicate alumina silicate Phase Sub Crystalline Rutil Rutil Rutil Rutil Rutil Phase Diameter of 0.05 0.05 0.05 0.05 0.05 Crystal Specific 3.04 3.07 3.04 2.96 3.00 Gravity (g/cm³) Yong's Modulus 107.85 107.85 110.85 114.85 112.85 Elastic 35.49 35.14 36.48 38.81 37.63 Modulus Ratio

TABLE 3 Example 11 Example 12 Example 13 Example 14 Example 15 SiO₂ 49.5 48.0 52.2 50.5 51.8 Al₂O₃ 24.2 29.4 22.5 19.2 24.1 MgO 19.9 15.1 18.0 18.9 17.2 P₂O₅ Nb₂O₅ Ta₂O₅ Li₂O TiO₂ 6.4 7.5 6.8 7.2 6.4 ZrO₂ 0.5 4.2 B₂O₃ 0.5 Y₂O₃ K₂O Sb₂O₃ ZnO La₂O₃ Melting 1500 1500 1500 1500 1450 Temperature (° C.) Melting Time 3.00 3.00 3.00 3.00 2.50 (hours) Primary 690 690 690 690 660 Treatment Temperature (° C.) Primary 5.50 5.50 5.50 5.50 5.00 Treatment Time (hours) Secondary 840 840 840 840 820 Treatment Temperature (° C.) Secondary 6.00 6.00 6.00 6.00 5.50 Treatment Time (hours) Main Magnesium Magnesium Magnesium Magnesium Magnesium Crystalline alumina silicate alumina silicate alumina silicate alumina silicate alumina silicate Phase Sub Crystalline Rutil Rutil Rutil Rutil Rutil Phase Diameter of 0.05 0.05 0.05 0.05 0.05 Crystal Specific 3.24 3.27 3.17 3.20 2.92 Gravity (g/cm³) Yong's Modulus 116.05 115.05 110.85 111.05 102.85 Elastic 35.83 35.19 34.98 34.71 35.23 Modulus Ratio

TABLE 4 Example 16 Example 17 Example 18 Example 19 Example 20 SiO₂ 50.2 52.0 50.0 48.2 48.0 Al₂O₃ 23.0 24.0 15.3 23.5 12.8 MgO 16.0 16.0 24.0 17.8 20.9 P₂O₅ Nb₂O₅ Ta₂O₅ Li₂O TiO₂ 7.6 6.2 6.5 6.8 7.8 ZrO₂ B₂O₃ 3.2 Y₂O₃ 1.8 4.2 K₂O 1.0 4.0 Sb₂O₃ 0.2 2.0 ZnO 0.5 2.0 La₂O₃ 2.0 2.5 Melting 1450 1500 1500 1450 1450 Temperature (° C.) Melting Time 2.50 3.00 3.00 3.00 3.00 (hours) Primary 660 690 690 660 660 Treatment Temperature (° C.) Primary 5.00 5.50 5.50 5.50 5.50 Treatment Time (hours) Secondary 820 840 840 820 820 Treatment Temperature (° C.) Secondary 5.50 6.00 6.00 6.00 6.00 Treatment Time (hours) Main Magnesium Magnesium Magnesium Magnesium Magnesium Crystalline alumina silicate alumina silicate alumina silicate alumina silicate alumina silicate Phase Sub Crystalline Rutil Rutil Rutil Rutil Rutil Phase Diameter of 0.05 0.05 0.05 0.05 0.05 Crystal Specific 2.92 3.20 3.24 3.24 3.27 Gravity (g/cm³) Yong's 102.85 116.85 118.85 109.85 112.85 Module Elastic Module 35.23 36.53 36.69 33.91 34.52 Ratio

Although the present invention has been fully described by way of example with reference to the accompanying drawings, it is to be understood that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modification depart from the scope of the present invention, they should be construed as being included therein. 

What is claimed is:
 1. A glass ceramic composition for recording disk substrate consisting essentially, expressed in terms of weight percent on an oxide basis, of from 47.5 to 52.5 wt % of SiO₂, from 10 to 30 wt % of Al₂O₃, from 10 to 30 wt % of MgO, and from 6.2 to 7.8 wt % of TiO₂, wherein the glass ceramic composition includes a main crystalline phase of magnesium alumina silicate crystal.
 2. A glass ceramic recording disk substrate, wherein the glass ceramic is prepared from a composition which consists essentially, expressed in terms of weight percent on the oxide basis, of from 47.5 to 52.5 wt % of SiO₂, from 10 to 30 wt % of Al₂O₃, from 10 to 30 wt % of MgO, and from 6.2 to 7.8 wt % of TiO₂, wherein the glass ceramic includes a main crystalline phase of magnesium alumina silicate crystal.
 3. A glass ceramic composition for recording disk substrate consisting essentially, expressed in terms of weight percent on an oxide basis, of from 50.0 to 52.5 wt % of SiO₂, from 10 to 30 wt % of Al₂O₃, from 10 to 30 wt % of MgO, and from 6.2 to 7.8 wt % of TiO₂.
 4. A glass ceramic recording disk substrate, wherein the glass ceramic is prepared from a composition which consists essentially, expressed in terms of weight percent on the oxide basis, of from 50.0 to 52.5 wt % of SiO₂, from 10 to 30 wt % of Al₂O₃, from 10 to 30 wt % of MgO, and from 6.2 to 7.8 wt % of TiO₂.
 5. A glass ceramic composition for recording disk substrate consisting essentially, expressed in terms of weight percent on an oxide basis, of from 47.5 to 52.5 wt % of SiO₂, from 10 to 30 wt % of Al₂O₃, from 10 to 30 wt % of MgO, and from 6.2 to 7.0 wt % of TiO₂.
 6. A glass ceramic recording disk substrate, wherein the glass ceramic is prepared from a composition which consists essentially, expressed in terms of weight percent on the oxide basis, of from 47.5 to 52.5 wt % of SiO₂, from 10 to 30 wt % of Al₂O₃, from 10 to 30 wt % of MgO, and from 6.2 to 7.0 wt % of TiO₂.
 7. A glass ceramics composition as claimed in claim 1, wherein said glass ceramic composition further consists essentially, expressed in terms of weight percent on an oxide basis, of from 0.1 to 5 wt % of P₂O₅.
 8. A glass ceramics composition as claimed in claim 1, wherein said glass ceramic composition further consists essentially, expressed in terms of weight percent on an oxide basis, of from 0.1 to 9 wt % of Nb₂O₅.
 9. A glass ceramics composition as claimed in claim 1, wherein said glass ceramic composition further consists essentially, expressed in terms of weight percent on an oxide basis, of from 0.1 to 9 wt % of Ta₂O₅.
 10. A glass ceramics composition as claimed in claim 1, wherein said glass ceramic composition further consists essentially, expressed in terms of weight percent on an oxide basis, of from 0.1 to 12 wt % of Li₂O.
 11. A glass ceramics composition as claimed in claim 1, wherein said glass ceramic composition further consists essentially, expressed in terms of weight percent on an oxide basis, of from 0.1 to 12 wt % of ZrO₂.
 12. A glass ceramics composition as claimed in claim 1, wherein said glass ceramic composition further consists essentially, expressed in terms of weight percent on an oxide basis, of from 0.1 to 5 wt % of B₂O₃.
 13. A glass ceramics composition as claimed in claim 1, wherein said glass ceramic composition further consists essentially, expressed in terms of weight percent on an oxide basis, of from 0.1 to 9 wt % of Y₂O₃.
 14. A glass ceramics composition as claimed in claim 1, wherein said glass ceramic composition further consists essentially, expressed in terms of weight percent on an oxide basis, of from 0.1 to 5 wt % of K₂O.
 15. A glass ceramics composition as claimed in claim 1, wherein said glass ceramic composition further consists essentially, expressed in terms of weight percent on an oxide basis, of from 0.1 to 5 wt % of Sb₂O₃.
 16. A glass ceramics composition as claimed in claim 1, wherein said glass ceramic composition further consists essentially, expressed in terms of weight percent on an oxide basis, of from 0.1 to 5 wt % of ZnO.
 17. A glass ceramics composition as claimed in claim 1, wherein said glass ceramic composition further consists essentially, expressed in terms of weight percent on an oxide basis, of from 0.1 to 5 wt % of La₂O₃.
 18. A recording disk substrate as claimed in claim 2, wherein said glass ceramic composition further consists essentially, expressed in terms of weight percent on an oxide basis, of from 0.1 to 5 wt % of P₂O₅.
 19. A recording disk substrate as claimed in claim 2, wherein said glass ceramic composition further consists essentially, expressed in terms of weight percent on an oxide basis, of from 0.1 to 9 wt % of Nb₂O₅.
 20. A recording disk substrate as claimed in claim 2, wherein said glass ceramic composition further consists essentially, expressed in terms of weight percent on an oxide basis, of from 0.1 to 9 wt % of Ta₂O₅.
 21. A recording disk substrate as claimed in claim 2, wherein said glass ceramic composition further consists essentially, expressed in terms of weight percent on an oxide basis, of from 0.1 to 12 wt % of Li₂O.
 22. A recording disk substrate as claimed in claim 2, wherein said glass ceramic composition further consists essentially, expressed in terms of weight percent on an oxide basis, of from 0.1 to 12 wt % of ZrO₂.
 23. A recording disk substrate as claimed in claim 2, wherein said glass ceramic composition further consists essentially, expressed in terms of weight percent on an oxide basis, of from 0.1 to 5 wt % of B₂O₃.
 24. A recording disk substrate as claimed in claim 2, wherein said glass ceramic composition further consists essentially, expressed in terms of weight percent on an oxide basis, of from 0.1 to 9 wt % of Y₂O₃.
 25. A recording disk substrate as claimed in claim 2, wherein said glass ceramic composition further consists essentially, expressed in terms of weight percent on an oxide basis, of from 0.1 to 5 wt % of K₂O.
 26. A recording disk substrate as claimed in claim 2, wherein said glass ceramic composition further consists essentially, expressed in terms of weight percent on an oxide basis, of from 0.1 to 5 wt % of Sb₂O₃.
 27. A recording disk substrate as claimed in claim 2, wherein said glass ceramic composition further consists essentially, expressed in terms of weight percent on an oxide basis, of from 0.1 to 5 wt % of ZnO.
 28. A recording disk substrate as claimed in claim 2, wherein said glass ceramic composition further consists essentially, expressed in terms of weight percent on an oxide basis, of from 0.1 to 5 wt % of La₂O₃. 